The intervertebral disc is an anatomically and functionally complex joint. The native intervertebral disc is made up of three component structures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3) the vertebral end plates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.
The spinal disc may be displaced or damaged due to trauma or a disease process. As a result of such displacement or damage, the nucleus pulposus may herniate and protrude into the vertebral canal or intervertebral foramen. That deformation is commonly known as a herniated or “slipped” disc. The deformation may press upon one or more of the spinal nerves exiting the vertebral canal through the partially obstructed foramen, causing pain or paralysis in its area of influence.
One method of alleviating this condition involves surgically removing the involved disc and fusing the two adjacent vertebrae. In this procedure, the removed disc is replaced by a spacer and secured to the neighboring vertebrae by screws, plates, and rods. Although “spinal fusion” evidences excellent short-term results, long-term studies show that the procedure eventually leads to degenerative changes in the spine, particularly at adjacent mobile segments. As a result of the fused segment's increased stiffness, adjacent discs incur increased motion and stress. In the long term, this change in the mechanics of the spine causes the adjacent discs to degenerate.
Prosthetic intervertebral discs are now used as alternatives to spinal fusion. Various artificial intervertebral disc designs are extant; many share the goal of mimicking the kinematics and load-sharing properties of the natural intervertebral disc. Two such design categories are ball-and-socket joint type discs and elastic rubber type discs.
Artificial discs of the ball-and-socket type usually include a pair of concave metal plates, one to be attached to the upper vertebra and the other to be attached to the lower vertebra, and a rounded core working as a ball. The concavities within the metal plates cooperate with and rotate with respect to the rounded core. The ball-and-socket type disc allows free rotation between the adjacent vertebrae between which the disc is installed. Such discs do not share any of the load placed on the spine as the spine bends.
In contrast, ball-and-socket discs have very high stiffness in the vertical (or compressive) direction, much higher than the normal compressive stiffness of the natural disc. As a result, although these discs allow flexion of the spine where a fused disc does not, the structure of these discs still causes adjacent discs to absorb extra compressive loads and still allow eventual and early degeneration of those discs.
Another common artificial disc design includes an elastic rubber or elastomeric polymer body embedded between a pair of metal plates. The disc is introduced into the emptied region between two adjacent vertebrae by affixing the two metal plates, via a surgical procedure, to those two vertebrae. The elastomeric polymer body is bonded to the metal plates through a rough, porous interface surface. This disc design is able both to absorb vertical, compressive shocks and to bear loads in that direction. However, the interface between the elastomeric polymer body and the metal plates is subject to peeling or severance due to the nature of the junction.
The prosthetic devices described here include a compressive core with gel or polymeric materials and a fiber-reinforced membrane forming the functional core periphery that, with proper application of our teachings, will match or approximate the functional characteristics of a healthy natural disc in its proper site in the spine.